Chewing gum: Imports are disallowed. I don't know about actually selling the stuff, since if you can't import it it's a moot point... Of course you can bring small amounts in for personal consumption - that's OK. But the customs officer has a lot of leeway to decide what constitutes "for personal consumption".
Spitting the gum on the ground would be considered littering. And as an addendum to what dandruff mentioned at the start, the ban on gum came into force soon after the train sytem opened.
Firearms and drugs are big no-nos... Zero tolerance there.
Carying around knives over a certain size is technically illegal. But there are times you can get away with it, for example: If you dive, chances are you'll have a dive knife, and people going on dive trips across the border are allowed to bring them through. But I've heard of people throwing a tantrum when the customs people question them about it... Not smart IMHO...
It isn't all uptight and law abiding though, plenty of drivers don't really care about the traffic laws. I have to cross a pedestrian crossing every day to get home, and its common to see people beating the red light there. Major junctions are usually worse...
